This invention relates generally to touchscreens, and more particularly, to touchscreens used to detect multiple touches.
Applications that have a need or requirement for sensing multiple touches are becoming more popular. For example, applications that use graphical interfaces to input information such as by displaying a standard keyboard on the screen may rely on a user selecting two keys at the same time, such as a shift key to make an upper-case character selection. Also, as screens become smaller, selecting multiple graphics at the same time may be desirable to provide input capability to a user within the smaller touchscreen footprint. Gaming and other multi-user applications also increase the need for detecting touches from two or more users interacting with the same touchscreen at the same time.
Some touch technologies, such as most resistive and capacitive touchscreens, may produce a single erroneous touch coordinate when subjected to simultaneous multiple touches. In contrast, surface acoustic wave (SAW) touchscreens and infrared (IR) touchscreens look at SAW and IR shadows, respectively, and produce a distinct shadow for each of the multiple simultaneous touches. SAW and IR touchscreens are therefore promising for multiple-touch applications, however, some ambiguity still exists in current SAW and IR touch technologies. A shadow-based touchscreen may receive two X coordinates and two Y coordinates for two touches, but may not automatically know which of the two X coordinates should be paired with which of the two Y coordinates. The processor may attempt to determine which coordinates should be paired based on graphics that were displayed at the time or by using other factors, such as a time of a lift-off event, but unfortunately these methods do not always produce reliable results.
Some current technology, such as some IR touchscreen designs using slightly non-parallel IR beams, may be able to discern multiple touches that are widely separated from each other, but are not able to discern multiple touches that occur in closer proximity to one another. Turning to SAW touchscreens, the problem of pairing X and Y coordinates has been addressed by measuring a third coordinate U. Such touchscreens may be referred to as “XYU touchscreens”. The coordinate U may be referred to as a rotated or diagonal coordinate and is linearly related to X and Y as follows in Equation 1, wherein A, B and C are geometrical constants.U=A*X+B*Y+C   Equation 1If a measured value of X is correctly paired with a measured value of Y, then the computed value of U that is based on the measured values of X and Y, will correspond to a measured value of U. In contrast, if measured X and Y values are incorrectly paired, then the computed value of U will not correspond to any measured value of U.
Unfortunately, known XYU touchscreen technology also has its limitations. To date, XYU touchscreen technology has not been applied to IR touchscreen designs because the narrow light emitting diode (LED) emission cone angle and the narrow phototransistor reception cone angle are incompatible with the common opto-electronic components being used for both X/Y and U beams. Known designs for XYU SAW touchscreens may require larger transmitters and receivers to send and receive acoustic signals due to the additional reflector arrays used to enable detection of multiple simultaneous touches. Also, additional transmit and receive U arrays require scattering by U array reflectors at angles significantly larger than 90 degrees, for which SAW scattering may be inefficient. This contributes to weak signals and acoustic parasites. In addition, SAW touchscreens are sealed to protect from liquid or other products which may come into contact with the touchscreen. The seal may absorb some of the acoustic energy, and it is not allowable to place a seal on top of an array. The seal may thus increase the overall size of the touchscreen or minimize the usable touch area of the touchscreen, both of which are undesirable.
Therefore, a need exists for improved XYU touchscreen designs for detecting multiple touches. Certain embodiments of the present invention are intended to meet these needs and other objectives that will become apparent from the description and drawings set forth below.